He when the UFO arrived.A. was sleeping B. were sleeping C. has slept

问题补充：

He when the UFO arrived.

A. was sleeping B. were sleeping C. has slept

答案：

解析：考查时间状语从句。句意“当UFO来时，他正在睡觉。”he是第三人称，故谓语用单数，排除B；根据arrived可知，用过去式，排除C。故选A。